The titration cell also consists of a smaller compartment with a cathode immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.

The Pt anode generates I2 when current is provided through the electric circuit. The net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is consumed for each mole of H2O. In other words, 2 moles of electrons are consumed per mole of water.

B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3

BSO3 + ROH → BH+ROSO3−

The end point is detected most commonly by a bipotentiometric method. A second pair of Pt electrodes are immersed in the anode solution. The detector circuit maintains a constant current between the two detector electrodes during titration. Prior to the equivalence point, the solution contains I− but little I2. At the equivalence point, excess I2 appears and an abrupt voltage drop marks the end point. The amount of charge needed to generate I2 and reach the end point can then be used to calculate the amount of water in the original sample.

The volumetric titration is based on the same principles as the coulometric titration except that the anode solution above now is used as the titrant solution. The titrant consists of an alcohol (ROH), base (B), SO2 and a known concentration of I2.

One mole of I2 is consumed for each mole of H2O. The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above.

The popularity of the Karl Fischer titration is due in large part to several practical advantages that it holds over other methods of moisture determination, including:

High accuracy and precision - typically within 1% of available water, e.g. 3.00% appears as 2.97 - 3.03%.

Selectivity for water

Small sample quantities required

Easy sample preparation

Short analysis duration

Nearly unlimited measuring range (1ppm to 100%)

Suitability for analyzing:

Solids

Liquids

Gases

Independence of presence of other volatiles

Suitability for automation

Linearity - single-point calibration, no calibration curves necessary

In contrast, loss on drying will detect the loss of anyvolatile substance.

The major disadvantage is that the water has to be accessible and easily brought into methanol solution. Many common substances, especially foods such as chocolate, release water slowly and with difficulty, and require additional efforts to reliably bring the total water content into contact with the Karl Fischer reagents.